The present invention relates to an SH wave generator designed to generate pure SH or transverse (shear) waves alone.
Investigations of underground structures by artificial seismic waves are crucial for both scientific purposes such as geophysical exploration and practical purposes such as ground surveying.
So far, P waves (longitudinal or compressional waves) have often been used as the seismic waves to this end. However, S waves also play an important role in making such investigations, because the elastic properties of matter vary with P and S waves. For instance, the P waves transmit through fluid zones, but the S waves do not, and bounce off the fluid-solid interfaces. For that reason, experts now believe that the S waves provide promising means for making a survey of what condition underground magmas are in or other investigations. It is also known that the concept of ground strength, important in terms of earthquake-proofness, is more affected by S wave velocity than by P wave velocity.
The S waves are broken down into SH waves causing wall particles to move in parallel to the surface of the earth and SV waves allowing soil particles to move vertically with respect to the surface of the earth. Upon reaching the underground fluid-solid interfaces, the SV waves are divided into reflected SV and P waves and refracted SV and P waves. By contrast, the SH waves only give rise to reflected SH waves and refracted SH waves when striking on the interfaces. In view of elastic waves, the SH waves are thus considered to be easier to manipulate than the SV waves, because their reflection and refraction mechanisms on the fluid-solid interfaces are relatively simple.
In order to make surveys as mentioned above by the S waves, an apparatus for generating the SH waves alone would be convenient to relay upon. So far, "SH wave cannon" and "plate vibration" techniques have been utilized for that purpose. There two types of apparatus are both designed to apply impactive shear forces to the earth's surface to generate SH waves.
However, a problem with the "SH wave cannon" technique is that although producing increased outputs, it is troublesome to handle. A difficulty with the "plate vibration" technique, on the other hand, is that it is easy to handle, but the energy to be produced is too low to achieve wide coverage.
For earthquake prediction, there is available the so-called vibrosize reflection technique, according to which a strong vibrator with a built-in hydraulic actuator is mounted on a motor vehicle for mobile seismic exploration. In this case, the frequency of the vibrator may be controlled on any desired time scale by a combination of electronics with hydraulic mechanism. For example, this may make is possible to generate the so-called sweep waves whose frequency varies with time. These input waves and the resulting reflected waves are well suited for data processing. With this technique, generally making use of P waves, some experiments have recently been attempted to generate S waves by oscillating the hydraulic actuator in parallel to the earth's surface. In this case, however, SV and P waves occur unavoidably in addition to the SH waves.